The invention relates to computer systems, and more particularly to a method and mechanism for prefetching in a computer system.
Many computer systems utilize servers, such as “database servers”, to store and maintain information. In a client-server computer system model (or a multi-tiered computer architecture), users that wish to access or modify information at the server are often located at a “client”. To facilitate the explanation of the invention, the terms “database server” and “database client” may be used in this document in place of “server” and “client”; however, the invention is not limited in its applicability to database systems, and indeed, can be utilized in many other types of computer systems.
In client-server systems, users at a database client submit commands to the database server to store, modify, or retrieve data. In response to the user commands, data manipulation or query activities are performed at the database server, with data results returned back to the database client for access. In networked environments, the database server often performs data manipulation or query commands submitted by remotely located clients. The client may establish a direct connection to the database server over the network, or may establish a connection through one or more intervening system components, such as an application server or transaction processing monitor. In either case, the database server processes the user commands and generates appropriate data outputs to be returned to the client. For example, a common database function is to perform data queries using a database query language such as SQL. The database server receives each query and generates a query result that satisfies the criteria defined by a particular query. The query result is subsequently transferred to the database client from which the query originated.
Inefficiencies may occur during the processing and transmission of data between the database server and client. For example, the database server produces a result set composed of a quantity of data that can be sent to a database client. The user may initially place an explicit request to transmit a first portion of that result set from the database server to the client, causing a first set of overhead, such as “network roundtrip overhead”, to be expended. At a later time, the user may request a second portion of the result set to be transmitted to the client, resulting in another set of overhead to be expended. This process may proceed until all of the result set is sent, resulting in multiple sets of roundtrip overhead to be expended between the database server and the client. The expense of sending the data in response to multiple requests also includes the wait time that is expended while the user waits for the request to be sent to the database server and for the subsequent data to be sent back to the client. In addition, if the transmitted data are broken into smaller pieces than the optimum data transfer sizes for the system, additional overhead is expended. Thus, the more pieces that the data set is broken into before transmissions from the server to the client, the greater the overhead that is likely to be expended.
Another type of inefficiency that may occur is the retransmission of data in the returned result set. If the data to be sent to the client contains redundancies, then excess overhead, such as increased transmission time and data storage at the client, is expended by the system to transmit and store that redundant data. When the amount of data redundancies is sufficiently large, the excess overhead can have a serious effect upon system performance. Such a circumstance may occur, for example, if the client is querying large database tables with sparse data for performing queries with joins involving wide tables.
As is evident from the foregoing, inefficiencies may occur as a result of data transmissions between a database server and a client. These same inefficiencies may arise from other types of client-server applications, such as for example, a web-based architecture for sending web pages from a web server to a client station having a web browser. Moreover, these inefficiencies may be exasperated by a query in which data that is accessed to respond to the query exists at a remote location. This may occur, for example, if the query is sent to be processed at a first server, but the first server needs to access one or more additional servers in order to adequately process the query.
The present invention is directed to a method and mechanism for implementing pipelined prefetching in a computer system. According to an embodiment of the invention, data prefetching is utilized to predictably retrieve information between multiple levels of nodes in the computer system, in which a first server may receive prefetched data from a second server, the second server receives prefetched data from a third server, with the chain of pipelined prefetching extending as long as needed/desired to effect efficient data transfers between the nodes. Further details of aspects, objects, and advantages of the invention are described below in the detailed description, drawings, and claims.